[llvm] [SSAUpdater] Don't use large SmallSets for IDFcalc (PR #97823)

[Jeremy Morse via llvm-commits]

jmorse wrote:

Looking back at my experiments, I originally hit a small regression (0.09% in -g builds) by using DenseSet, with the overall 0.13% improvement coming when the reserve call was added. So I guess it's the reserves.

SmallPtrSet doesn't have a reserve method right now, but if you reckon it's a better fit here then we can add one.

https://github.com/llvm/llvm-project/pull/97823